Thanks for the question Deb! There are no simple answers for this one. The reasons have been accumulating for almost 200 years. But I am so glad you asked. As I was thinking on how best to respond, it turned into a full article. That will include some history and the basis for my answers.
You are correct. One can't just go to a reservation and say "Where are your Elders? I want them to teach me." That would be offensive on many levels. Knowledge is shared as one becomes ready, open to learn and accept it. Worthiness and purity of intentions are also important factors.
One must establish themselves as a worthy pupil, and this can take some time. The best way to start is through networking and becoming a "friend" to an Indian community. Attend their public functions and meet Tribal members. Let them get to know you. Start out asking general information and learn about their history. Always be respectful and understanding if told something is private, not to be shared.
Learn and observe their rules and customs. Apologize quickly and sincerely for goofs, and readily admit you are like a child just beginning its education. We might be experts in our fields outside of the Indian community. But with an Elder, we are children. I've made this mistake, interjecting on something I was knowledgable about. However, to the Elder it was like a 5 year old instructing them how to conduct a Sweat Lodge. While I thought it was helpful, it was actually disrespectful. I am still working on keeping my mouth shut and my ears open.
Help out where and when possible. Not monetarily, but with yourself. Asking "may I come" or "may I do this" is especially appreciated.
After one has become known, the questions come. I've experienced this several times, a non-chalant, but very probing question asked. ALWAYS listen carefully and answer all questions truthfully from the heart. These are tests and subterfuge will be detected immediately. This skill of detection has been honed and perfected over the centuries. I've also had some very blunt questions asked of me. It all depends on the personality. Some Elders test the waters gently with a new person, and others get straight to the point.
As long as one is pure of heart and intention, they will become a welcomed friend. The teaching will begin when that friend is ready. Once the teaching begins, then we can politely ask for more.